The present invention relates to a separator for a valve regulated lead acid battery and a valve regulated lead acid battery employing the same.
A conventional separator for a valve regulated lead acid battery contains fine glass fibers as the main component. With such a separator containing fine glass fibers as the main component, a short circuit between a positive electrode plate and a negative electrode plate can be caused due to the mechanical cause I or the electrochemical cause II as follows:
I. Mechanical cause: When there is projection(s) (grids, granular balls of active material or the like) on either electrode plate, local pressure or shear force is applied to the separator by the projection(s). If the strength of the separator yields the given pressure or force, the separator is penetrated or ripped so that the projection comes in contact with the other electrode, thus causing the short circuit.
II. Electrochemical cause: As electrolyte becomes closer to pure water because sulfate ion in the electrolyte is consumed at the final stage of discharge, lead ion becomes more soluble so that lead sulfate generated on positive and negative electrodes dissolves partially. When the battery is charged after the generation of lead sulfate, the lead ion in the electrolyte is reduced at the negative electrode so that metallic lead is deposited. Therefore, dendrites are grown in the separator to reach the other electrode plate, thus causing the short circuit.
As a separator which can prevent such a short circuit from occurring due to the aforementioned causes, a separator has been proposed which is composed mainly of fine glass fibers and includes inorganic powder such as silica powder and natural pulp (JP2000-268796A, JP2001-185115A).
JP2000-268796A discloses a separator which is composed mainly of fine glass fibers (preferably, fine glass fiber having a mean fiber diameter of 1 μm or less) and includes from 5 to 30% by weight of inorganic powder (preferably silica particles having a specific surface area of 100 m2/g or more) and from 3 to 20% by weight of beaten natural pulp (preferably, natural pulp having a Canadian freeness of 250 L or less). The density of the separator is equal to or more than 0.165g/cm3.
Since the separator includes the inorganic powder such as silica, the separator has pores of small diameter and therefore has densified structure, thereby preventing an electrochemical short circuit from occurring due to the electrochemical cause II. Since the separator includes the beaten natural pulp, the separator has higher tensile strength and high punctual resistance, thereby preventing a mechanical short circuit from occurring due to the mechanical cause I.
A separator disclosed in JP2001-185115A is composed mainly of fine glass fibers and includes from 5% to 20% by weight of pulp and from 1% to 15% by weight of inorganic powder (preferably, silica powder) of which particle diameter is 50 pm or less. JP2001-185115A also discloses a valve regulated lead acid battery having the separator and electrode plates which are arranged to have an interval of 1.0 mm or less.